The wurtzite GaN has either Ga or N polarity. The direction of polarization, hence it¡¦s associated polarization-induced electric fields (Fp), is determined by the polarity of the sample. In the present work, we prepared both N-face polarity and n-type doping of GaN with thicknesses are 1.1£gm and 70nm. Photoreflectacne (PR) and contactless electroreflectance (CER) were used in combination to study the nature of the surface band bending which was found to be determined by the type of doping for the thick sample and by the polarity for the polarity for the thin sample. This is in agreement with a theoretical calculation by Poisson-Schrödinger solver. Hence, CER can determine the polarity of GaN film as long as the sample is thin enough of the Fp to become dominant in the surface region.